1. Field of the Invention
This invention relates generally to high-pressure fluid conducting hoses and particularly to a hose structure incorporating a plurality of reinforcing layers.
2. Prior Art
There are known numerous reinforced pressure hoses for conducting operating fluids for boring drilling or other construction machineries. Such hoses are wound on reels when not in use and taken out or unreeled at the site of construction. The hose is thus repeatedly subjected to bending and longitudinal tensile forces as well as frictional forces applied to the cover tube.
One of the prior art reinforced pressure hoses is depicted in FIG. 9 of the accompanying drawings in which the hose 100 measuring about 25 mm in diameter comprises a rubber core tube 101, a first reinforcing layer 102, an intermediate rubber layer 103 interposed therebetween, a second reinforcing layer 104 and a rubber cover tube or sheath 105. The first and second reinforcing layers 102 and 104 are braided or knitted helically with metallic wires at a stationary bias angle (hereinafter defined) of approximately 109.5.degree.. With such stationary bias angle, the hose would undergo an elongation in tension of as much as 8% or greater against an applied load of 250 kgf or greater than 40% against a load of 1,000 kgf.
Another prior art hose is disclosed in Japanese Utility Model Publication No. 51-42095 in which the hose features the use of a wire braided reinforcing layer knitted at a stationary bias angle of 109.5.degree. for combatting internal pressure applied to the core tube and a yarn braided reinforcing layer knitted at a bias angle (hereinafter defined) of 60.degree.-90.degree. and having a yarn elongation at break of 8%-14% for protecting the hose against tensile forces exerted longitudinally thereof. The latter reinforcing layer would contribute to objectionably increased hose elongation due to intrinsic limitation of the yarns in resistance to tensile load and further to damage of the cover tube from increased frictional forces when the hose is stretched in its lengthwise direction.
A further prior art hose is disclosed in Japanese Utility Model Publication No. 1-33904 wherein there is shown a reinforced pressure hose comprising a plurality of metal wire reinforcing layers disposed between a core and a cover tube and intermediate rubber layers interposed between adjacent reinforcing layers. One of the reinforcing layers adapted to reinforcement against internal operating pressure has a stationary bias angle of 109.5.degree., and the other reinforcing layer assigned to reinforcement against tensile load has a bias angle of between 80.degree. and 100.degree.. A hose of this type is highly flexible and hence easy to take up on a reel, but susceptible to excessive elongation as when exposed to a tensile load of say greater than 5 tons, leading to increased permanent stretch set. Attempts to reduce the extent of elongation by selecting a bias angle of smaller than 80.degree. for the tensile reinforcement layer would result in too much elongation of the hose, making it harder to wind up into a roll.
A still another prior art illustrated in FIG. 10 is directed to a hose 200 comprising a rubber core tube 201, a first reinforcing layer 202, a second reinforcing layer 203, a rubber intermediate layer 204 interposed therebetween, a third reinforcing layer 205, a rubber layer 206 disposed between the second and third reinforcing layers, and a rubber cover tube 207. The first, second and third reinforcing layers are formed of a braid or helical knit of metal wires interlaced at bias angles of .alpha.x, .alpha.y and .alpha.z, respectively. Since the bias angle .alpha.z for the third reinforcing layer 205 is greater than the bias angles ax and .alpha.y for the first and second reinforcing layers 202 and 203, this bias angle relationship would result with the third or outermost reinforcing layer 205 restraining the tendency of the first and second reinforcing layers 202, 203 to expand upon removal of the hose from a mandrel. As illustrated in FIG. 11, during mandrel removal the third reinforcing layer 205 will vary in its bias angle from .alpha.z to .alpha.z' corresponding to a radial expansion of S.sub.2, whereas the first or second reinforcing layer will shift in bias angle from .alpha.x to .alpha.x' corresponding theoretically to a radial expansion of S.sub.1. However, in the prior art hose of FIG. 9 where the bias angle relation is .alpha.x&lt;.alpha.z and hence the radial expansion relation is S.sub.2 &lt;S.sub.1, the tendency of the first and second reinforcing layers 202, 203 to expand to a value of S.sub.1 is in fact hindered or restricted to a value of the difference between S.sub.1 and S.sub.2 (i.e., S.sub.1 -S.sub.2) as the result of the third reinforcing layer having a knit bias angle greater than that of the first or second reinforcing layer. Thus, the hose 200 is rendered difficult to draw out from the mandrel around which it has been wrapped.